This invention relates to apparatus for automatically handling batches of small disk-like elements, such as semiconductor wafers, and is more particularly directed to a system for transferring a batch of wafers between one slotted carrier to another slotted carrier.
In the process of semiconductor wafers to make semiconductor devices, the wafers are subjected to a myriad of processing steps. It is practical to perform many of these steps on a batch of wafers positioned in a carrier having a plurality of spaced slots for receiving the wafers in edgewise, coaxial relation with the wafers spaced from each other. Many of these steps may be performed in a carrier or cassette made of plastic, which is relatively inexpensive and easy to work with. However, other steps must be performed at high temperatures, such as in a heat treating furnace, at temperatures that cannot be tolerated by the plastic cassette. Consequently, it is common practice to transfer the wafers from a plastic cassette to a carrier made of quartz, often referred to as a boat. Also, it is sometimes desirable to transfer the wafers from the quartz boat back to the plastic cassette.
The wafers can, of course, be manually transferred, but this is a delicate, tedious process, perhaps requiring the use of a tweezers, with the result that the delicate wafers are often damaged or contaminated through handling. If the damage is immediately detected, this is a serious loss simply because of the cost of the partially processed wafers. If the damage or contamination is not detected until later, in the form of an unacceptable end product or system in which the product is used, the loss is greatly amplified.
As an attempt to improve the method of handling wafers, the open side of a plastic cassette may be mated with the open side of a quartz boat with the slots of one being carefully aligned with the slots of the other and the wafers then being transferred by pushing the wafers into the boat slots and then inverting the carriers. This approach also has the disadvantage of being a somewhat difficult operation that can result in damage to the delicate and expensive wafers.
One of the difficulties of attempting to transfer batches of wafers from one carrier to another carrier is that the width of the vertical slots in some cassettes is such that the wafers can tilt randomly and are not spaced uniformly or are not precisely parallel. Wide spaces are desired for ease of inserting or withdrawing wafers; which makes the tilting problem more difficult.
In yet another approach, individual wafers have been automatically transferred one by one from one carrier to another. This system is, of course, also very time-consuming. Other automated systems have been attempted; however, all leave something to be desired.
Accordingly, a need exists for an improved automated wafer transfer apparatus with which a batch of wafers may be safely but efficiently transferred.